EEVblog Electronics Community Forum

Electronics => Beginners => Topic started by: sy on March 01, 2024, 10:14:45 am

Title: Mesh Analysis
Post by: sy on March 01, 2024, 10:14:45 am
Hello,
I am struggling with mesh analysis and would like some help. I have seen Dave's video explaining the topic and attempted to apply it to a circuit from another guy's video. I saw comments saying that the guy did it wrong, so I simulated it on LTSpice and the people in comments were indeed correct.

https://www.youtube.com/watch?v=8f-2yXiYmRI (https://www.youtube.com/watch?v=8f-2yXiYmRI)
https://www.youtube.com/watch?v=eQpc2QRFv7Y (https://www.youtube.com/watch?v=eQpc2QRFv7Y)

The equations I have set up for mesh 1 and mesh 2 are:
3 - 2(I1) - 1(I1-I2) = 0        --(1)
I2 = -1.5A                           --(2)

And from that I get that I1=0.5A (the same as the guy's working).

Would someone be able to help point out where I went wrong? Apparently there is no current running through R3, and it's just the one big outer loop running 1.5A through the circuit anticlockwise.

Many thanks!
Title: Re: Mesh Analysis
Post by: Ian.M on March 01, 2024, 10:21:55 am
You are missing a connection.  LTspice (and other good schematic editors) places a dot where wires join to indicate a connection.   The wire between bottom of R3 and GND is not connected to the wire between the bottom of the sources, so of course there is no current through R3.
Fix: Select wire drawing mode.  Put the cursor on one of the crossing wires near the joint.   Drag to make a new wire, ending at the crossover point.  LTspice will join the wires and add the dot.
Title: Re: Mesh Analysis
Post by: sy on March 01, 2024, 10:29:15 am
BRILLANT and also embarrassing hahaha. Didn't see that one coming. Thankyou Ian.
Title: Re: Mesh Analysis
Post by: Zero999 on March 01, 2024, 10:43:06 am
It's generally bad practice to draw a schematic with four wires connected together like that.

Use DC op pnt for simple DC simulation, rather than transient analysis. It's much faster and will give you a nice table showing the voltages and currents in all parts of the circuit. Data labels showing the voltages can be inserted, by clicking on the nodes in the circuit.

[attachimg=2]
Code: [Select]
       --- Operating Point ---

V(n002):	 2	 voltage
V(n001):	 3	 voltage
V(n003):	 8	 voltage
I(I1):		 1.5	 device_current
I(R3):		 -2	 device_current
I(R2):		 1.5	 device_current
I(R1):	 	-0.5	 device_current
I(V1):	 	-0.5	 device_current
Title: Re: Mesh Analysis
Post by: sy on March 01, 2024, 11:00:53 am
Thankyou Zero, I will keep that in mind!

Also, with the .op command, how do I know which voltage n002 or n001 correspond to in the circuit at a glance? Is there a way to assign the names at each node?

I clicked on the nodes without the command running and it showed me the corresponding voltages. However, those voltages become "? ? ?" when I run the simulation, and I don't seem to be able to click on the nodes in the circuit with the .op command running.
Title: Re: Mesh Analysis
Post by: Ian.M on March 01, 2024, 11:03:45 am
Quote from: Zero999 on March 01, 2024, 10:43:06 am
It's generally bad practice to draw a schematic with four wires connected together like that.
+1.  Back in the old days of manual drafting and opto-mechanical photocopiers, crossing lines often developed a splodge so a straight cross as a junction was strictly forbidden.  Nowadays your EDA tools will let you draw such a junction, but as you've found, its still a trap for the unwary (especially if an image of the schematic is compressed with lossy compression).  See below for alternatives.

Quote from: sy on March 01, 2024, 11:00:53 am
Also, with the .op command, how do I know which voltage n002 or n001 correspond to in the circuit at a glance? Is there a way to assign the names at each node?

Quote from: sy on March 01, 2024, 11:00:53 am
I clicked on the nodes without the command running and it showed me the corresponding voltages. However, those voltages become "? ? ?" when I run the simulation, and I don't seem to be able to click on the nodes in the circuit with the .op command running.
You cant click nodes while the .op results are open as the sub-window is modal.  Also the new .op results are not available to the schematic so all .op data lables show as '???' (unknown).  Close the results window and you can hover any node or component to access the node number/label or component ID (instance name), and previous .op result for that node/component.  If you need to refer back to the full results *without* re-running the .op sim, do menu 'View' 'Visible traces' to reopen the results window!  Alternatively, before closing the results window, do Ctrl-A Ctrl-C to select and copy them and paste them into a text document.
Title: Re: Mesh Analysis
Post by: Zero999 on March 01, 2024, 11:11:43 am
Quote from: sy on March 01, 2024, 11:00:53 am
Thankyou Zero, I will keep that in mind!

Also, with the .op command, how do I know which voltage n002 or n001 correspond to in the circuit at a glance? Is there a way to assign the names at each node?

I clicked on the nodes without the command running and it showed me the corresponding voltages. However, those voltages become "? ? ?" when I run the simulation, and I don't seem to be able to click on the nodes in the circuit with the .op command running.
By default, nodes are named Nxxx where xxx is a sequential number, which depends on the order the schematic has been entered in. Node names are displayed in the left hand side of the status bar, when the mouse cursor is placed over a node.

The nodes can be named, with the label net command.

Unfortunately, you need to close the simulation in order to click on the nodes and for the data labels to display the voltages, rather than question marks, which is a pain. A work around is to copy the contents of the simulation by pressing alt+a, then ctrl+c and pasting it into notepad.
Title: Re: Mesh Analysis
Post by: watchmaker on March 01, 2024, 11:55:53 am
I just finished my course(S) on DC analysis.  Self Study. Used Real Analog, the Real Analog Text and Labs, and the Boylestad DC Analysis book (and labs).  AND I also checked my exercises and lab work against bread board and LT Spice. (See my post in the sticky above on learning materials)

I needed all these tools.  The real Analog offers only simple examples.  But it is good for an overview.  And it is not clear that at some point he is using matrix math.  The Boylestad corrects both of these.  He has many examples, good labs, explains matrix math, LT spice, Matlab, and the TI89.

Began last fall and just started RA video 14. Maybe I am a slow learner?  But to learn matrix math, regression, MatLab, simulators, TI89, breadboarding (which IS a manual skill to master) on top of network analysis is a lot to take in.

Maybe (I do like the RA stuff) the Boylestad alone is effective. 

The big issue is redrawing the networks to reflect the new reality when you start shorting and opening sources. And to have enough experience to recognize how to simplify weirdly drawn networks.  It is a spacial thing.

As far as I am concerned, good on ya!  I think this network analysis is pretty tough to learn by self study.
Title: Re: Mesh Analysis
Post by: CaptDon on March 01, 2024, 01:54:18 pm
I second that observation where straight crosses were not allowed due to photocopy issues where some non-connected crosses ended up looking connected and some connected crosses ended up looking non-connected. I have run into this many times and most recently with a poor quality copy of a Heathkit manual. In all of my personal schematics of my projects I have always used the 'horse shoe loop' for non-connected crosses!! Mesh theory, I solved those problems with Thevinin or Norton, I guess that is mesh-in-a-box? Always got the correct answer. The real bugger is A.C. with multiple power sources and loads with huge phase offset.